Fabrizia Grepioni

Mechanochemical preparation of molecular and supramolecular organometallic materials and coordination networks

This Dalton Perspective deals with solvent-free reactions taking place within solids or between solids or involving a solid and a vapour. The focus is on reactions involving organometallic and coordination compounds and occurring via reassembling of non-covalent bonding, e.g. hydrogen bonds, and/or formation of ligand-metal coordination bonds. It is argued that reactions activated by mechanical mixing of solid reactants as well as those obtained by exposing a crystalline solid to a vapour can be exploited to make crystals, which is the quintessence of crystal engineering. It is demonstrated through a number of examples that solvent-free methods, such as co-grinding, kneading, milling of molecular solids, or reactions of solid with vapours represent viable alternative, when not unique, routes for the preparation of novel molecular and supramolecular solids as well as for the preparation of polymorphic or solvate modifications of a same species. The structural characterization of the products requires the preparation of single crystals suitable for X-ray diffraction, a goal often achieved by seeding.

Unprecedented mechanochemical preparation of 18Crown[6] and 15Crown[5] adducts of ammonium hydrogen sulfate by grinding or kneading

The complex 18Crown[6]·[NH4][HSO4]·2H2O has been prepared by co-grinding of solid 18Crown[6] ether and solid [NH4][HSO4], while the complex (15Crown[5])3·[NH4]2[H3O][HSO4]3·H2O has been obtained by kneading liquid 15Crown[5] together with solid [NH4][HSO4]. Structural characterization has been achieved on single crystals obtained by recrystallization of the solid products from water.

Mechanochemical and solution reactions between AgCH3COO and [H2NC6H10NH2] yield three isomers of the coordination network {Ag[H2NC6H10NH2]+}∞

Solid-state co-grinding of silver acetate and solid trans-1,4-diaminocyclohexane, [H2NC6H10NH2] yields two isomeric coordination networks depending on the crystallization conditions; a third isomeric form is obtained when the same reaction is carried out in solution.

Gas–solid reactions between the different polymorphic modifications of barbituric acid and amines

The two known crystalline polymorphic forms of barbituric acid (C4H4N2O3) (form I and form II) and the dihydrate form [(C4H4N2O3)·2H2O] have been reacted with vapours of ammonia, methylamine, and dimethylamine and the crystalline products investigated by means of single crystal and powder X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry and infrared spectroscopy. It has been shown that form I, II and the dihydrate form of barbituric acid react with ammonia leading to the same crystalline ammonium barbiturate salt NH4(C4H3N2O3) 1a, while the gas–solid reactions of form II with methylamine, and dimethylamine yield the corresponding crystalline salts, CH3NH3(C4H3N2O3) 1b and (CH3)2NH2(C4H3N2O3) 1c. Thermal desorption of the bases at ca. 200 °C leads to formation of a new crystal form of barbituric acid, form III, as confirmed by H1-NMR spectroscopy and by chemical behaviour. De-hydration of the dihydrate form has also been investigated showing that it releases water to yield exclusively crystals of form II.